OK – I can hear you objecting – I can see how this 'no crossing branches'
constraint can be made to work mechanically, but what on earth is the point
of it? Why should it matter whether branches can or cannot cross? A good
question (if you asked it for the right reasons, and not just because you're be-
ginning to get exasperated with Syntax!) Well, the motivation behind this con-
straint is essentially as follows: on general metatheoretical grounds (the desire
to develop a maximally constrained theory which provides a plausible basis
for developing a model of language acquisition), we want to restrict the class
of P-markers which qualify as 'possible natural language sentence-structures'
as narrowly as possible, and rule out as many 'impossible' structures as we can.